The investigation of the thorium fuel cycle is a collaborative INPRO (International Project on Innovative Nuclear Reactors and Fuel Cycles) activity within the main area of global vision of sustainable nuclear energy for the 21st century. The current publication reports on the sustainability of nuclear power by re-examining the potential of thorium based fuel cycles to support future large scale deployment of nuclear energy systems by increasing the availability of nuclear material. Special attention is paid to the thorium fuel cycle from the point of view of economics and proliferation resistance.